Feedback control systems are well known and have many practical uses. They can be very simple, or they can be quite sophisticated and complex. Many have analog or digital computers used to perform the control function, have sensors for sensing process or plant conditions, and have outputs for controlling the process or plant. Most broadly, a feedback control system can be thought of as having a command input, a feedback (or plant) input, control elements for sensing the relationship between command and the feedback signals and producing a controller output in accordance with the error between those signals, for controlling the process. Feedback control systems can be as simple as the thermostat in a home heating system. In the automobile both the cruise control and the anti-lock braking system can include closed loop feedback control systems.
The present invention deals with reasonably sophisticated control systems which, in addition to responding to command and feedback signals to produce a controller output as a result of differences between the command and feedback signals, are subjected to and are intended to compensate for non-linear "disturbances". A significant application for such control systems is for fuel control valves (or injector valves) for large industrial engines, such as turbines or multiple cylinder internal combustion engines. In such systems an actuator position is controlled to a particular point to allow fuel to flow at a particular rate, and the feedback system will sense an engine parameter (such as RPM, for example) and compare that with an input command signal to maintain the position of the actuator at a point which will minimize the error signal. Most often, except for transient conditions, the valve is stationary. If an adjustment is required the control system must produce a signal to drive the valve to its new position. However, friction comes into play, and it is known that before the valve begins to move, a force must be applied which is adequate to overcome static friction. That force is typically significantly greater than the force it would take to simply move the valve to its new position if friction were not present. Sliding friction forces must also be overcome when the valve is moving. It is these types of friction effects which can be considered non-linear disturbances in this example.
It will be noted at this point that friction is not universally considered to be a disturbance by all. However, in feedback control system such as those disclosed in the present application, it is a useful concept for control purposes. Accordingly, friction will be treated as a disturbance, and in the preferred embodiment, the disturbance of interest in this application.
Those skilled in the art will appreciate that the disturbance signal can be ignored and ultimately with a properly configured control loop, the control and feedback signals will ultimately drive the system to the desired position, but commonly with limit cycles due to friction. However, that is only acceptable in system which can tolerate slow response times. When a highly responsive control, which has a fast response time and resulting high bandwidth is required, it is not acceptable to simply ignore the disturbance signal and allow the disturbance to be subsumed in the control. That will typically lengthen response times to beyond acceptable limits. Instead, steps must be taken to somehow compensate or make provision for the expected non-linear disturbance. This is preferably done in a way which does not hinder overall system performance and response time, and also in a way which does not detrimentally affect the stability of the system.
In the past, "dither" techniques have been used in an effort to deal with disturbances such as these. A dither signal is a noise signal, e.g. a square wave or pulse signal which is injected into the system in an effort to "overpower" the disturbance signal, and thus its effect. The magnitude of the dither signal is great enough to overcome the friction. The effect of the dither is to continuously oscillate the output signal so that the actuator is being driven (only a minuscule amount) in one direction or the other, such that if an error signal requiring valve movement is produced by the controller, the dither will have already overcome static friction. This technique has a number of disadvantages. The dither is, of course, non-adaptive, and thus the magnitude must be set to a level higher than the largest disturbance intended to be encountered. In the friction example, it must be set higher than the highest level of friction which the system is expected to encounter. Furthermore, friction is a varying non-linear force, and the dither does not have the ability to adapt to the differing friction levels. Consequently, the use of a dither approach may provide too much or too little force to overcome the frictional forces. In addition, the dither reduces system robustness, through excitation of system resonances, or by making the control response less determinant.
Suggestions have been made as to adaptive ways of handling disturbances when disturbances are of the linear variety. However, when the disturbances become non-linear, resort has been had to other techniques, such as the use of the dither, in order to approach acceptable system response times and steady state performance.